Method and apparatus for estimating frequency

ABSTRACT

A method and apparatus for use in connection with wireless communication to adjust the frequency of an oscillator to synchronize with a received signal by correlating a synchronization code channel with training sequences to estimate relative offsets which are employed to estimate an error, which is then filtered. The filtered output preferably provides a voltage controlling a voltage controlled oscillator (VCO). The same technique may be employed to control a numeric controlled oscillator (NCO).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/438,595, filed Apr. 3, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/185,361, filed Aug. 4, 2008, now U.S. Pat. No.8,149,963, which issued on Apr. 3, 2012; which is a continuation of U.S.patent application Ser. No. 11/754,013, filed May 25, 2007, now U.S.Pat. No. 7,412,013, which issued on Aug. 12, 2008; which is acontinuation of U.S. patent application Ser. No. 11/088,116, filed Mar.23, 2005, now U.S. Pat. No. 7,236,547, which issued on Jun. 26, 2007;which is a continuation of U.S. patent application Ser. No. 10/629,429,filed Jul. 29, 2003, now U.S. Pat. No. 7,187,732, which issued on Mar.6, 2007; which claims the benefit of U.S. Provisional Application No.60/399,818 filed on Jul. 31, 2002, which are all incorporated byreference as if fully set forth.

TECHNICAL FIELD

This application relates to wireless communication and wireless devices.More particularly, the invention relates to initialization of acommunication link between a base station (BS) and a user equipment(UE).

BACKGROUND

During an initial cell search (ICS) or power-up of a UE, a trainingsequence of known symbols is used by the receiver to estimate thetransmitted signal. In a time division duplex (TDD) signal, for example,the midamble of a TDD frame conventionally contains the trainingsequence of symbols. The conventional cell search process consists of aStep 1 algorithm which processes a primary synchronization code (PSC) onthe primary synchronization code channel (PSCH) for synchronizationchannel (SCH) location determination. A Step 2 algorithm processes thesecondary synchronization codes (SSC) for code group determination andtimeslot synchronization, and a Step 3 algorithm performs midambleprocessing.

Variable control oscillators (VCOs) are commonly used at the end of anautomatic frequency control (AFC) process to adjustably control thefrequency of the receiver to achieve synchronization between atransmitter and a receiver. The input for the VCO is a control voltagesignal, which is typically generated by a control circuit that processesthe amplitude and phase of the received symbols. A common problem duringan AFC process is the initial fluctuations resulting from a potentiallysignificant frequency offset between the transmitter and the receiver.

SUMMARY

A method and apparatus for use in connection with wireless communicationto adjust the frequency of an oscillator to synchronize with a receivedsignal by correlating a synchronization code channel with trainingsequences to estimate positive and negative offsets which are employedto estimate an error, which is then filtered. The filtered outputpreferably provides a voltage controlling a voltage controlledoscillator (VCO). The same technique may be employed to control anumeric controlled oscillator (NCO).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood from the following description anddrawings in which like elements are designated by like numerals and,wherein:

FIG. 1 is a block diagram showing the phase rotation approach forstart-up AFC.

FIGS. 2A and 2B, taken together, comprise a block diagram of theinteraction between start-up AFC and algorithm Steps 1, 2 and 3 of cellsearch.

FIG. 2 shows the manner in which FIGS. 2 a and 2 b are arranged tocreate a complete block diagram.

FIG. 3 shows a process diagram for a PI filter.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a start-up adaptive frequency control (AFC)10 used to reduce the frequency offset between a base station (BS) anduser equipment (UE) during initial cell search procedure. Start-up AFCuses a phase rotation approach, which is based on the correlations oftwo sequences with the primary synchronization code (PSC). The storedPSC sequence 12 is rotated in opposing directions at 14, 14 a, 16, 16 ato respectively determine correlations with the received sequence 18 at20 and 22. The absolute values (a and b) are obtained at 24 and 26 andto obtain the value

${\left( \frac{a - b}{a + b + c} \right)6\mspace{14mu} {kHz}},$

from circuit 27, where c is an arbitrary constant provided to preventdivision by zero. The phase rotation at −3 kHz alternatively can bereplaced by a conjugate of a rotated PSC sequence at 3 kHz since the PSCsequence can only have values of (1+j) and (−1−j).

During start-up AFC process, it is assumed that the PSC locationprovided is correct. Once Step 1 completes generation of the firstoutputs, the start-up AFC starts running. The Step 1 process andstart-up AFC process run in parallel. Optimally, start-up AFC reducesthe frequency offset from 6 kHz to less than 2 kHz in the least numberof iterations. Table 1 shows a particular advantage of frequencycorrection which is an increase in allowable integrations. The number ofintegrations is limited, however, due to chip slip. The chip-slip upperboundary is 0.5 Tc since the maximum correlation is generated one samplelater for a method utilizing twice the chip rate sampling. Table 1summarizes the allowable number of integrations as frequency offset isreduced. Table 2 provides information on performance degradation for acoherent combining technique in the presence of carrier frequencyoffset.

TABLE 1 Frequency Offset vs. Number of Integrations Allowed Slip perFrequency Offset frame Number of integrations allowed ±6 kHz = ±3 ppm0.1152 Tc 4 ±4 kHz = ±2 ppm 0.0768 Tc 6 ±2 kHz = ±1 ppm 0.0384 Tc 13  ±1kHz = ±0.5 ppm 0.0192 Tc 26

TABLE 2 Frequency Offset vs. Code Length for Coherent Combining Lengthof the code integrated Carrier frequency Offset Loss in dB coherently Fc= 2 GHz 2.42 256 ±3 ppm 6 kHz 1.04 256 ±2 ppm 4 kHz 0.26 256 ±1 ppm 2kHz 0.06 256 ±0.5 ppm   1 kHz 12.62 512 ±3 ppm 6 kHz 4.53 512 ±2 ppm 4kHz 1.04 512 ±1 ppm 2 kHz 0.26 512 ±0.5 ppm   1 kHz

The start-up AFC procedure includes a mechanism to realign the primarysynchronization code (PSC) position that may shift during correction.The Step 1 procedure can be run to eliminate the need for the mechanismwhile the start-up AFC algorithm is running. The Step 1 procedureupdates the peak location every 4^(th) frame.

FIG. 2 depicts the parallel processing relationship among start-up AFCand Steps 1, 2 and 3 of cell searching. Of particular concern is therelationship between Step 1 and start-up AFC. Since Step 1 works inparallel with the startup AFC, there is no need for a code trackercircuit to follow a given path. Each time Step 1 updates an output thatis based on the largest detected value, start-up AFC uses the new peaklocation to estimate the new frequency offset.

The frequency estimator block (FEB) 31 of the start-up AFC comprises aSequence Locator and Splitter 32, frequency estimators 34-38, aproportional plus integral (PI) filter 42, and a voltage controlledoscillator (VCO) or numeric controlled oscillator (NCO) 46 coupled to PIfilter 42 through the sign flop 44. The input 32 a to the SequenceLocator and Splitter 32 includes the PSC peak location chip-offsetprovided by Step 1. Start up AFC 30 is an open loop gain control blockthat steps through pre-defined gain levels in order to set proper inputpower level before digitizing the input. The main input to both Step 1and the Sequence Locator and Splitter 32 is sampled at twice the chiprate with a length of 76,800 complex elements. Since the chip-offsetpoints to the peak location, the beginning of the PSC is 511 samplesbefore the chip-offset. The outputs of the Sequence Locator and Splitter32 are generated by the following general equation:

Output=input[i−511]i  Eq. (1)

Accordingly, the three particular outputs of the Sequence Locator andSplitter 32 are represented by the following equations for early (32 b),punctual (32 c) and late 32(d) estimates:

Early[i]=input[i−511]i=offset−1,offset,offset+1, . . . ,offset+510  Eq.(2)

Punctual[i]=input[i−511]i=offset,offset+1,offset+2, . . .,offset+511  Eq. (3)

Late[i]=input[i−511]i=offset+1,offset+2,offset+3, . . . ,offset+512  Eq.(4)

Although the Locator and Splitter 32 in the example given in FIG. 2, isa PSC locator, it should be understood the same approach can be usedwith any received sequences other than PSC.

The input samples to the Sequence Locator and Splitter are taken attwice the chip rate.

The frequency estimators 34, 36 and 38 each receive one of the threeinputs provided by Equations (2)-(4). The frequency estimators estimatea different frequency offset, summed at 40, for each input sequence inaccordance with FIG. 1. The frequency offset, summed at 40, is thesummation of early, punctual and late estimates.

The sum of the estimates is passed through a proportional plus integral(PI) filter 42 with coefficients alpha and beta, respectively as shownin detail in FIG. 3. The PI filter bandwidth has two settings.Initially, alpha and beta are preferably ½ and 1/256, respectively asshown in detail in FIG. 3. The loop gain k is set at (k=−1.0). Duringsteady state, alpha and beta are set to 1/16 and 1/1024, respectively.FIG. 3 depicts such a PI filter structure 42. The preferable settingsfor coefficients alpha and beta are summarized in Table 3. However,other filters may be substituted for the PI filter.

TABLE 3 PI Filter Coefficients as a Function of Operating Conditions.Condition alpha beta initial ½ 1/256 steady state 1/16 1/1024

Steady state condition is established when:

the startup AFC completes at least ten (10) iterations;

while the last eight (8) outputs (inputs to VCO) are put into a bufferof length eight (8); the difference between the absolute value of theaverage of the first half and that of the second half is within ±1 kHz;and

the current output to the VCO is within ±1 kHz of the absolute value ofthe average of the second half.

For digital applications, a numerically controlled oscillator (NCO) isused in place of the VCO.

The start-up AFC algorithm relies on PSC location update to estimate thecarrier frequency offset. Step 1 runs during frequency correction toupdate the PSC location. As such, it is preferable that start-up AFC isbegun immediately following a successful Step 1 process, with Step 1running in parallel. Step 1 continues to provide updated PSC locationsonce every N1 frames as per the Step 1 algorithm, where N1 is themaximum number of frames for averaging. Start-up AFC is run in thismanner for a duration of L frames, with L=24 as the preferred value. TheStep 1 FLAG 61 from controller 60 is set when a sequence is detected.The FEB 31 runs when the controller 60 provides an enable condition toFEB 31 at 62. Since the peak locations shift left or right in time, theStep 1 algorithm is run constantly. At the end of L frames, the start-upAFC reduces the frequency offset to about 2 kHz in many cases, whichprovides considerable enhancement to the Step 2 performance. Theinclusion of L frames contributes to the overall cell search delaybudget and hence is chosen conservatively to be L=24.

PSC processing block 66 correlates against the primary synchronizationcode in (synchronization channel) (SCH) over frames. The SCH location isnot known.

SSC extractor block 68 utilizes the SCH location and extracts only theSCH portion, which is then passed to SSC processing block 70.

SSC processing block 70 correlates against the secondary synchronizationcode in synchronization channel over SCH.

Midamble Extractor block 72 utilizes the SCH location and SSC processingresults and extracts the midamble portion to pass to midamble processingblock 74.

Midamble processing block 74 correlates against possible midambles givenby SSC processing and picks the one with the highest energy.

Periodic Cell Search block 76 performs a process which constantlysearches for the best base station for the given period.

Controller 60 coordinates among stages to synchronize to a base station.

Layer 1 Controller 80 coordinates all layer 1 related hardware andsoftware in order to maintain proper operation in the receiver.

What is claimed is:
 1. A method of estimating frequency, the methodcomprising: rotating a phase of a stored sequence to produce a firstversion of a stored sequence having a rotated phase; correlating areceived sequence with the first version of the stored sequence toproduce a first phase correlation; correlating the received sequencewith a second version of the stored sequence that has a differentrotated phase than the first version to produce a second phasecorrelation; and combining the first and second phase correlations toprovide the frequency estimate.
 2. The method of claim 1 furthercomprising: initial cell search processing a received sequence tofacilitate the determination of a frequency location of a receivedsignal by: producing a first frequency estimate based on an earlyversion of the received sequence, a second frequency estimate based on apunctual version of the received sequence, and a third frequencyestimate based on a late version of the received sequence; combining thefirst, second and third frequency estimates; and filtering the combinedfrequency estimates.
 3. The method of claim 2 wherein the initial cellsearch processing is repeated every N frames where N is an integergreater than
 0. 4. The method of claim 2 wherein the stored sequence isa primary synchronization code sequence.
 5. The method of claim 2further comprising repeating the rotating, correlating and combining agiven number of times to produce a series of frequency estimates foreach of the first, second and third frequency estimates.
 6. The methodof claim 2 wherein a received input power level of the received signalis adjusted prior to the rotating, correlating and combining.
 7. Themethod of claim 6 wherein the received signal is digitized afteradjustment of the power level and the power level is set employing openloop gain control.
 8. The method of claim 2 wherein the initial cellsearch processing includes obtaining a primary synchronization code. 9.The method of claim 8 further comprising employing the primarysynchronization code to extract a secondary synchronization code fromthe received signal.
 10. The method of claim 2 further comprising usingthe filtered combined frequency estimates to control the generation of afrequency by a numerically controlled oscillator.
 11. The method ofclaim 2 further comprising using the filtered combined frequencyestimates to control the generation of a frequency by a voltagecontrolled oscillator.
 12. A wireless communication device comprising: afiltering component; and a frequency estimator configured to rotate aphase of a stored sequence to produce a first version of a storedsequence having a rotated phase, correlate the received sequence withthe first version of the stored sequence to produce a first phasecorrelation, correlate the received sequence with a second version ofthe stored sequence that has a different rotated phase than the firstversion to produce a second phase correlation, and combine the first andsecond phase correlations to provide the frequency estimate.
 13. Thewireless communication device of claim 12 further comprising an initialcell search processing component configured to process a receivedsequence to facilitate the determination of a frequency location of areceived signal by producing a first frequency estimate based on anearly version of the received sequence, a second frequency estimatebased on a punctual version of the received sequence, and a thirdfrequency estimate based on a late version of the received sequence;combining the first, second and third frequency estimates; and filteringthe combined frequency estimates with the filtering component.
 14. Thewireless communication device of claim 13 further comprising a sequencelocator associated with the initial cell search processing component andconfigured to selectively produce received signal input for thefrequency estimator.
 15. The wireless communication device of claim 13wherein the frequency estimator is configured to use is a primarysynchronization code as the received sequence.
 16. The wirelesscommunication device of claim 13 wherein the filtering component isconfigured to selectively integrate frequency estimates responsive to aninitial or steady state conditions of a cell search process.
 17. Thewireless communication device of claim 13 wherein the filteringcomponent is a Proportional integral (PI) filter.
 18. The wirelesscommunication device of claim 13 further comprising an oscillatorconfigured to produce an adjusted frequency during initial cell searchusing the filtered combined frequency estimates.
 19. The wirelesscommunication device of claim 18 wherein the oscillator is a voltagecontrolled oscillator (VCO).
 20. The wireless communication device ofclaim 18 wherein the oscillator is a numeric controlled oscillator(NCO).